1. Field of the Invention
The present invention relates to a method of and an apparatus for controlling the quality of processed seismic data.
2. Description of the Related Art
Seismic data processing generally involves taking the raw acquired seismic data through a series of processing steps to produce a finally processed seismic image. The finally processed seismic image is then geologically interpreted in order to make decisions on the hydrocarbon prospectivity within the seismic survey area; the closer the finally processed seismic image is to the perceived geology, the better the quality of the processed seismic data.
The seismic processing steps and parameter selection for these steps are, therefore, very dependent on geology and target. In order to calibrate the final image in absolute terms, a priori knowledge of the geology would be required. However, such knowledge is not always available.
Current practice today is as follows: at each processing step, those parameters which can be varied are tested on a portion of seismic data. This involves producing a series of consecutive seismic image panels for each value of the "tested" parameter which are displayed either in hardcopy or on a workstation graphics display. The "best" panel is then chosen from a visual inspection and the parameters associated with that panel are selected as "optimum" for that processing step for the rest of the data. The selection of the "best" panel may be aided by the production of other supplemental information which is used to provide an indication of the "quality" of the data. For example, in the case of deconvolution (see later), autocorrelation plots and the impulse response may be used in order to select the "best" panel. However, this technique is very subjective and time consuming and does not enable the geophysicist to fully understand the "quality" of the processed seismic data. This often causes an unknown degradation in the final processing quality because of incorrect choice of parameters.
An example of a seismic processing route could include the following known steps:
1. Designature--The shape of the input energy source signature is converted to one with a known property which improves the performance of successive processing steps.
2. Gather--The seismic data are reordered so that energies reflecting at the same point of the subsurface are grouped together. These are commonly called CMP's (Common Mid Points).
3. Velocity Analysis--The data within the CMP's will contain information from varying source-to-receiver offsets (distance between source and receiver). The time at which a given subsurface interface is recorded is a function of both the source-to-receiver offset and the velocity through the subsurface. The time delay of the interface with offset is exploited to determine a velocity profile within the subsurface.
4. Deconvolution--Primary energy is that energy which travels from the source, reflects from a sub-surface interface and returns directly to the receiver, i.e. it represents the desired earth response. Unfortunately spurious events called multiples occur where energy reflects more than once between interfaces.
These spurious or multiple reflections are reduced through deconvolution. The deconvolution process can also compress the time series wavelet which represents any given interface and as such is an aid to increasing the resolution of closely separated interfaces.
5. Stack--The velocity profile derived from step 3 is used to correct all the recorded offset data to simulate source/receiver coincident data. These corrected traces are then added together to enhance the "primary" signal at the expense of ill-corrected or non-primary energy such as noise or multiples.
6. Migration--An assumption made within the gather and stack processing steps is that all the subsurface horizons are horizontally bedded. The migration process moves any non-horizontal layers to their correct spatial positioning as well as focusing the seismic image.
7. Filtering--Any unwanted frequencies not considered as primary reflection energy are removed.
Each one of these steps has a set of input parameters which affects the quality of the output data after processing at that step. For example, the parameters of the deconvolution test are selected to meet two criteria: to reduce the multiples content within the data; and to increase the resolution of the seismic image in order to distinguish separate interfaces. As the deconvolution process is statistically driven by the seismic data, the choice of the deconvolution operator parameters will control both the compression of the seismic wavelet and the periodicity of the multiples that will be attenuated. Typically, three parameter selection tests are carried out:
(i). Deconvolution window design--to decide which portion of the seismic data will be used statistically to drive the deconvolution process;
(ii). Deconvolution active operator length--to decide which multiple periods are best reduced,
(iii). Deconvolution gap--to decide the amount of wavelet compression or resolution at the interfaces.
As mentioned hereinbefore, parameter selection has previously relied upon the skill and experience of an operator and has therefore, to a large extent, been subjective. The actual effects of such selection have not been known and inappropriate selection of parameters could only be judged subjectively by "dissatisfaction" with the processed seismic image. Reprocessing the seismic data with different parameters is very costly and, in any case, would involve further subjective assessment of unpredictable consequence.